There used to be the joy of Tanaka Day. Now doomsday is around the corner.

There once was Harvey Day and then there was Tommy John surgery.

Masahiro Tanaka appears to be heading down the same pitching path.

The Yankees got the news Thursday night, announced by general manager Brian Cashman, that Masahiro Tanaka will be out for at least six weeks with a small tear in his right ulnar collateral ligament and will begin an extensive rehab process.

If that doesn’t work, Tommy John surgery is the next step. Then all bets are off on the Yankees’ $175 million investment. It’s a new Tanaka World.

The bottom line is if Tanaka were in this kind of shape before the Yankees signed him, they never would have given him the kind of money they shelled out.

Not only have they lost their ace for at least six weeks and maybe much longer, but there now will be more of a burden on the pitching staff. That means extra work for bullpen bulldogs Dellin Betances and David Robertson.

Joe Girardi has to be careful the rest of the best arms on his team don’t blow out.

The Yankees’ pitching fell apart Thursday night in an ugly 9-3 loss to the Indians at Progressive Field, a game the Yankees led 3-0 going into the seventh. Starter David Phelps gave up two runs and then Matt Thornton ran into all kinds of trouble, followed by Jim Miller’s blowup. The Indians scored four in the seventh and five in the eighth.

If all goes well with Tanaka, and the Yankees somehow hang in the race in the mediocre AL East, getting Tanaka back in six weeks (at best) will be a plus, but don’t expect the same Tanaka who dominated until his recent troubles.

Since the Mike Napoli home run on June 28, Tanaka has allowed 20 hits and 10 runs over 13 ²/₃ innings.

That is not an ace.

Clearly, Tanaka was hurting because of the small tear, a tear that is less than 10 percent of the ligament, but will he have the same stuff he had before the injury?

No way.

Now there will be the cloud hanging over his head every time he goes to the mound. Now Girardi is going to have to be extra cautious with Tanaka.

Three doctors recommended the rehab protocol — Chris Ahmad (Yankees), David Altchek (Mets) and Neal elAttrache (Dodgers), so that is the way to go. But because of the stress Tanaka has put on his right arm coming over to the major leagues with the bigger baseball and throwing those 477 split-finger fastballs this season, he is no longer a sure thing.

“If we knew today that the best course of action was Tommy John surgery,’’ Cashman said of the doctors’ recommendation, “despite the name and the amount in the investment, we would be told Tommy John surgery.

“Hopefully no more than six weeks. Time will tell. It’s a disappointing situation and one that none of us wanted to be talking about or experiencing.’’

If Tanaka does not respond well to the rehab, surgery will be done and it will be at least a year for Tanaka to return.

This is the same no man’s land the Mets were in with Matt Harvey last summer. Harvey eventually decided to have the surgery and 2014 went out the window. The rest of 2014 is a question mark for Tanaka and if it gets worse, scratch 2015 off your calendar.

When Joba Chamberlain had his Tommy John surgery, his UCL was completely torn.

“You can’t sit around and feel sorry for yourself,’’ Derek Jeter said of the Yankees’ injury woes.

They have lost their entire starting rotation, except for Hiroki Kuroda.

“The back end of the bullpen has saved them,’’ one scout said of the Yankees. “But now with Tanaka down, they could get a couple of bad starts and go into a tailspin real fast.’’